For example, a thin-film transistor (TFT) is included in a liquid crystal panel or an organic EL panel. A channel portion of the thin-film transistor is made of amorphous silicon a-Si, or polycrystalline silicon poly-Si which is a crystalline material. A crystalline silicon layer (poly-Si layer) of the channel portion of the thin-film transistor is typically produced by forming an amorphous silicon layer (a-Si layer) and then irradiating the amorphous silicon layer with a laser beam of an excimer laser or the like so that the amorphous silicon layer is instantly increased in temperature and crystallized.
There are two types of thin-film transistor structures, namely, a bottom-gate structure in which a gate metal is located on a substrate side as seen from x-Si (x is a or poly) of the channel portion, and a top-gate structure in which a gate metal and a source-drain metal are located on a side opposite to the substrate side as seen from x-Si of the channel portion. The bottom-gate structure is mainly used for an a-Si TFT with a channel portion formed of an amorphous silicon layer, whereas the top-gate structure is mainly used for a poly-Si TFT with a channel portion formed of a crystalline silicon layer. The bottom-gate structure is commonly used as a structure of thin-film transistors in a liquid crystal panel or an organic EL panel used for a large-area display device.
There is also an instance where the poly-Si TFT has the bottom-gate structure, which provides an advantage of a reduced manufacturing cost. In the poly-Si TFT having the bottom-gate structure, the crystalline silicon layer is formed by irradiating an amorphous silicon layer with a laser beam to crystallize the amorphous silicon layer. In this method (laser annealing crystallization), the amorphous silicon layer is crystallized by heat generated by laser beam irradiation.
As a method of laser annealing, there is a method in which a silicon oxide layer is deposited as a buffer layer on the amorphous silicon layer, for example, and a light absorbing layer is deposited on the buffer layer, and the light absorbing layer is irradiated with a laser beam absorbed by the light absorbing layer and converted into heat so as to heat the amorphous silicon layer indirectly. This method is hereafter referred to as an indirect heating method with laser.
As the laser used for the indirect heating method with laser, a fixed laser in red or near infrared region capable of achieving high output and having output highly stable in time is effective. This is because, if the intensity of the laser beam changes in time, the temperature distribution is not uniform for crystals, resulting in non-uniform crystallinity in the crystalline silicon layer formed by the crystallization. With the excimer laser, it is difficult to form uniform crystals due to a problem such as difference in energy (change in time). Furthermore, the fixed laser is advantageous in terms of manufacturing in that the maintenance cost is reduced compared to the excimer laser which is a gas laser.
As the light absorbing layer used for the indirect heating method with laser, it is preferable to have optical characteristics with which absorptance for light having a wavelength in red and near-infrared region, more specifically, from 600 nm to 2000 nm is high. Furthermore, the light absorbing layer also has thermal characteristics, which makes it capable of going through a laser annealing crystallization process at a high temperature.
Examples of the light absorbing layer with the characteristics include Mo and Cr which are metals with high melting points. The metal films with high melting points generally have a large extinction coefficient k (greater than or equal to 2). Accordingly, with a film thickness (greater than or equal to 10 nm) that allows the metal films to be formed stably and going through the heating by the laser irradiation (greater than or equal to 1500 degrees), the transmittance of the metal film for the incident laser beam is less than or equal to 5%. Accordingly, the influence of multiple interference on the layered structure underneath can be ignored. Thus, regardless of the layered structure underneath, (for example, depending on the region in which the gate electrode is present and the region in which the gate electrode is not present), the absorptance of the light absorbing layer is constant.
Thin-film transistors in an organic EL panel are required to have particularly uniform characteristics. Accordingly, applying the aforementioned laser annealing crystallization to manufacturing of the thin-film transistor of the bottom-gate structure has the following drawback (problem). More specifically, in the thin-film transistor of the bottom-gate structure, first a gate electrode is formed using a metal material of higher heat conductivity than silicon or an insulation film, and then an insulation layer and an amorphous silicon layer are formed. Furthermore, after the light absorbing layer is formed on the amorphous silicon layer formed, the light absorbing layer which is the upper layer is irradiated with the laser beam, and the amorphous silicon layer is annealed so as to crystallize the amorphous silicon layer indirectly with the heat by the indirect heating method with laser. Upon the crystallization, there is a problem that the heat that was supposed to be used for crystallizing the amorphous silicon layer is absorbed by and propagated by the gate electrode, consequently causing lowered or non-uniform crystallinity due to insufficient crystallization of the amorphous silicon layer.
In view of such a problem, there is disclosed a method of disposing a dummy gate pattern in a nearby region of the gate electrode, i.e. a channel neighborhood, to reduce a difference in heat capacity between the amorphous silicon layer located above the gate electrode and the amorphous silicon layer located above the dummy gate pattern (for example, Patent Literature 1). There is also disclosed a method of extending the gate electrode to a laser beam scan upstream side so that, through the use of a pre-annealing effect of the extended portion of the gate electrode, the gate electrode is thermally saturated before laser beam reaches the light absorbing layer above the gate electrode of the thin-film transistor, thereby keeping the gate electrode from absorbing heat generated in the silicon thin film (for example, Patent Literature 2).